It is well known to align optical components on a silicon substrate using register surfaces as mechanical stops. In this approach, an optical device, such as a laser, is precisely aligned to a silicon wafer surface by butting a precision notch surface of the laser against a mechanical stop or pedestal protruding from the silicon wafer board surface. The height of the laser or optical device is determined by the height of a separate etched standoff positioned under the laser on the wafer surface. An optical fiber, intended to be optically aligned to that laser, is registered to the silicon wafer board by bonding it to a precision-etched V-groove. Because the optical components are aligned to the silicon wafer board, they are aligned to each other. This alignment process is referred to herein as “physical passive alignment” or “physical alignment.”
Although physical alignment offers relatively high accuracy in aligning optical components on a substrate, the applicants have identified two major disadvantages of the approach which limit its accuracy. First, the formation of the V-groove is made traditionally by a crystallographic wet etch which is very sensitive to the orientation of the wafer crystal and is constrained in shape. Indeed, the sole purpose of the crystallographic etch is to expose slow etching 111 silicon wafer crystal surfaces that then can be used for holding parts mechanically. Unfortunately, commercially-available silicon wafers are provided with orientation markings which have a significant tolerance, e.g., +/−0.5°. If an etching mask, for the purpose of etching a V-groove trench, is placed on the surface with the trench out of crystal alignment by even 0.50°, the trench will etch deeper than expected and the centerline of the trench will shift out of position making the substrate for holding the optical components slightly imprecise.
The second major disadvantage of the present process is that the mask for defining the V-groove trench is not on the same mask level as the mask used for defining the fiducials for positioning the laser. This means that there is additional error introduced if the two separate masks are not precisely aligned to each other's position in subsequent fabrication steps in the wafer board's fabrication.
Therefore, the applicants have identified a need for an optical substrate having precision alignment features which are unaffected by crystalline misalignment and avoid tolerance build up of multiple mask application steps. The present invention fulfills this need among others.